1. Field of the Invention
The present invention relates in general to a video cassette recorder, and more particularly to a chrominance signal playback system and method, which reduce the generation of noise on the screen while the video cassette recorder is in the playback mode.
2. Description of the Prior Art
Referring to FIG. 5, there is shown, in block form, an example of a conventional video signal playback system for a video cassette recorder, referred to hereinafter as a VCR, which includes luminance signal and chrominance signal processing circuits. In this figure, composite video signals are detected by video heads 51 and 52 which then apply the detected video signals to an amplifier 53 which amplifies the video signals by predetermined degrees of amplification. The amplified video signals from the amplifier 53 are applied to luminance signal and chrominance signal processing circuits 54 and 55 which processes the video signals, respectively, in such manners as to obtain luminance and chrominance signals from the video signals. These processes are well-known in the VCR art. Therefore, these processes will hereinafter be described in brief.
The luminance signal processing circuit 54 comprises a high-pass filter 56 which high-pass filters the video signal from the amplifier 53 to separate a luminance signal from the video signal on the basis of frequency band. The resulting luminance signal is demodulated into an original luminance signal by a demodulator 57 which then applies the demodulated luminance signal to a low-pass filter 58. In result, the low-pass filtered output from the low-pass filter 58 is a desired luminance signal. This luminance signal is then applied to a mixer 59. The output from the low-pass filter 58 is also applied to a synchronous signal separator 60 wherein synchronous pulse signals at horizontal and vertical frequencies are separated and applied to the chrominance signal processing circuit 55.
The chrominance signal processing circuit 55 comprises a low-pass filter 61 which low-pass filters the video signal from the amplifier 53 to separate a chrominance signal from the video signal on the basis of frequency band. The resulting chrominance signal is applied to an automatic gain controller 62 for automatic control of its gain. The output from the automatic gain controller 62 is demodulated into an original chrominance signal by a demodulator 63 which then applies the demodulated chrominance signal to a color killer circuit 64. The automatic gain controller 62 performs the automatic gain control of the resulting chrominance signal from the low-pass filter 61 according to the feedback chrominance signal through a color burst gate 65 and a gain detector 66. The output from the color killer circuit 64 is applied to a band-pass filter 67. In result, the band-pass filtered output from the band-pass filter 67 is a desired chrominance signal. This chrominance signal is combined with the luminance signal from the luminance signal processing circuit 54, in the mixer 59. Finally, the mixer 59 outputs the resulting composite video signal.
The color killer circuit 64 is a circuit for blocking the chrominance signal output when there is no play-back chrominance signal, or when the play-back chrominance signal is weak, in order to prevent noise generation. In operation, a pulse generator 68 is driven by the synchronous signals from the synchronous signal separator 60, for generating pulses at a predetermined period. In accordance with the pulses from the pulse generator 68, a color burst gate 69 performs a gate operation to extract a color burst signal. The output from the color burst gate 69 is applied to a killing signal detector 70 which controls the color killer circuit 64 according to whether the color burst signal is present or not. That is, if no color burst signal is present or if the color burst signal has a level lower than a given reference value, the killing signal detector 70 controls the color killer circuit 64 to block the input of the play-back chrominance signal to the mixer 59. As a result, under the control of the killing signal detector 70, the color killer circuit 64 blocks the output from the demodulator 63 to the band-pass filter 67. This results in the general absence of the appearance of white noise generated in the chrominance signal processing circuit, in the video output.
The color burst signal is sometimes referred to as a color synchronizing signal. In the VCR, it is necessary to make the local oscillator frequency and phase concurrent with the chrominance subcarrier frequency and phase, so as to extract a red signal-luminance signal (R-Y) and a blue signal-luminance signal (B-Y) from the chrominance carrier. For this reason, in recording the video signal the color synchronizing signal is included. The color synchronizing signal is generally referred to as the color burst signal. The color burst signal is typically inserted in a back porch portion of the horizontal synchronous signal which is present in a horizontal blanking interval of the composite video signal, as shown in FIG. 6.
However, the conventional video signal playback system has the disadvantages described below. Generally, in the luminance signal processing circuit, the conventional system has used a line noise removing circuit utilizing a video signal line correlation and a noise suppressor circuit utilizing a pre-emphasis circuit and a de-emphasis circuit. However, in the chrominance signal processing circuit, such a noise suppressor circuit has not been employed since the frequency band of the chrominance signal to be processed is narrower than that of the luminance signal.
Also, the color killer circuit 64 in the chrominance signal processing circuit 55 in FIG. 5 is desirable in order to block or pass the chrominance signal output in accordance with the presence and level of the color burst signal, but has a disadvantage described below. Namely, the color killer circuit does not perform the killing operation when a monochrome signal is a portion of the composite video signal and the color burst signal is normally present in every field. This results in the intermixture of white noise from the chrominance signal processing circuit into the monochrome signal portion of the composite video signal, resulting in degradation in image quality of the play-back video signal on the screen and degradation in the signal-to-noise ratio thereof. Generally, the case where a portion of the television broadcasting video image on the screen is monochrome is rare. However, since the level of the chrominance signal is feeble or weak on a dark portion of the screen, the level of the white noise becomes relatively higher than that of the chrominance signal on that portion. This means that it is impossible to avoid the above-mentioned problem in the prior art system.